As is conventionally well known, efficient power generation by a proton-exchange membrane fuel cell desirably involves keeping an electrolyte membrane in a moderately wet condition and preventing a water content inside the fuel cell from becoming deficient or excessive. An example of a known technique for controlling the water content in a cell plane of a fuel cell is described in Patent Document 1 (Japanese Patent Laid-Open No. 2004-335444). Patent Document 1 discloses controlling the distribution of water content as droplets or water vapor in a cell plane by adjusting at least one of pressure, humidity, temperature, and flow rate of a reactant gas (a general term for oxidation gas typified by air and fuel gas typified by hydrogen gas) and pressure drop characteristics defined by flow channel geometry.